In typical well formation for many oil and gas wells, a well bore is drilled and then a metal casing is forced down the borehole with sections of casing being joined to one another. Once the casing is in place the outside of the casing is filled with cement, at least to a certain well depth, to effectively the seal the casing from the surrounding rock and ensure that, in use, the only flow path is through the casing. Once the cement has cured the well is typically perforated by lowering a ‘gun’ which comprises one or more shaped charges to a desired depth of the well bore.
When the perforation charges are fired the shaped charges perforate the casing, cement and rock bed in the direction that the charge is facing and thus create a flow path from the reservoir into the well. In some well formations the perforations may be stimulated, for instance by hydraulic fracturing or acidization to increase flow, and then production equipment, filters, sand screens, production tubing and the like may be fitted. A similar process may be used in some injection wells, for instance for sequestration of unwanted and/or hazardous materials.
In some well formations optical fibres are deployed down the wellbore to be used for sensing purposes. For example patent application WO2010/136773 discusses use of an optical fibre deployed downwell to provide distributed acoustic sensing (DAS) downwell
Fibre optic distributed acoustic sensing (DAS) is a known technique whereby a single length of optical fibre is interrogated, usually by one or more input pulses of light, to provide substantially continuous sensing of acoustic activity along its length. Optical pulses are launched into the fibre and the radiation backscattered from within the fibre is detected and analysed. By analysing the radiation backscattered within the fibre, the effect of acoustic signals incident on the fibre can be detected. The backscatter returns are typically analysed in a number of time bins, typically linked to the duration of the interrogation pulses, and hence the returns from a plurality of discrete sensing portions can be separately analysed. Thus the fibre can effectively be divided into a plurality of discrete sensing portions of fibre. Within each discrete sensing portion disturbance of the fibre, for instance from acoustic sources, cause a variation in the properties of radiation which is backscattered from that portion. This variation can be detected and analysed and used to give a measure of the intensity of disturbance of the fibre at that sensing portion.
As described in WO2010/136773 the fibre optic cable may be attached to the outside of the casing as it is forced into the wellbore and then cemented in pace during the cementing step. It is also known to provide distributed temperature sensing using a downwell optical fibre and again this fibre may be located on the outside of the production casing.
One problem that arises with this approach is that the optical fibre is in situ during the perforation step. Were a perforation charge to be fired in the direction of the optical fibre, the perforation step could sever, or at least severely damage, the fibre at that location with the result that no useable optical signal can be received from the optical fibre at locations deeper into the well. As the well may be perforated at several sections along it length, damage to the optical fibre at a section towards the top of the well could mean that no useable signals may be received from the section of fibre deployed in the production zone. It will of course be understood that as the optical fibre is clamped in pace to the casing and cemented in place replacing a damaged optical cable is not a viable option.
The perforation gun, which typically contains several shaped charges and may have shaped charges directed in various different directions, may therefore oriented before firing to avoid the optical fibre. However at a perforation depth which may be a kilometer or more and could be several kilometers the relative location of the fibre may not be known, and it can also be problematic to accurately orient the perforation charges.
It is known therefore to clamp the optical fibre in relation to a metallic feature on the casing, for instance the fibre may clamped next to a metallic object, such as a metal rod which is also clamped to the outside of the casing. The perforator gun containing the shaped charges may then be provided with a magnetic anomaly detector which is connects to the surface. The readout from the magnetic anomaly detector may therefore be used to determine the orientation of the perforator gun with respect to the metallic feature and hence the optical fibre.
It has been found however that such magnetic anomaly detection techniques are not always satisfactory and the magnetic signature may be masked in the downhole environment with the result that the perforator can be incorrectly aligned when fired and the optical fibre has been damaged.
Alignment of a perforator gun with respect to the optical fibre is particularly important as incorrect alignment may result in damage to the optical fibre. However there may well be other tools that are deployed downwell, for instance via wire line, where knowing the orientation of the tool may be useful and where magnetic anomaly detection may be insufficiently accurate.
It is therefore an object of the present invention to provide methods and apparatus for orienting objects, especially perforators, downwell which at least mitigate some of the aforementioned disadvantages.